1. Field of the Invention
This invention relates to masks for lithography and more particularly to phase-shift masks for use in photolithography.
2. Description of Related Art
In photolithography, masks are employed to expose a pattern upon a work piece. As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques which permit enhancement of the current performance of the process of photolithography. One approach is to use phase-shifting techniques in the ranges of wavelengths used in photolithography in the past.
At present, small features or small geometric patterns are created by using conventional optical photolithography. Typically, optical photolithography is achieved by projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Once the light areas and the dark areas are formed, they are projected onto and through a lens and subsequently onto a substrate. However, because of increased semiconductor device complexity which results in increased pattern complexity, and increased pattern packing density on the mask, distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore, making diffraction a severe limiting factor for optical photolithography.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase-shift mask, which replaces the previously discussed mask. Generally, with light being thought of as a wave, phase-shifting is a change in timing of a shift in wave form of a regular sinusoidal pattern of light waves that propagate through a transparent material. Typically, phase-shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes or both, thereby changing the phase or the periodic pattern of the light wave. Phase-shift masks reduce diffraction effects by combining both diffracted light and phase-shifted diffracted light so that constructive and destructive interference takes place favorably.
One type of phase shift mask, as well as a detailed description of theory is disclosed in Marc D. Levenson et al., "Improving Resolution in Photolithography with a Phase-Shifting Mask," I.E.E.E. Transactions on Electron Devices, Vol. ED-29, No. 12, December 1982.
In his paper, "Phase-Shifting and Other Challenges in Optical Mask Technology, (short course on phase-shift mask technology, SPIE Conference, 1991) Burn J. Lin discusses a variety of phase-shifting techniques, including alternate phase shifting in which every other element in a closely packed array is phase-shifted, sub-resolution phase shifting which serves to enhance the edge contrast of patterns below the resolution limit of the given optical system, rim phase shifting in which phase shifting takes place solely at the rim of the patterns, and others. Lin tested the various types of phase shifting on five different feature patterns. All five features patterns were improved by rim phase shifting.
In addition to alternate phase shifting, there is another technique known as attenuated phase shifting wherein a mask is provided that includes absorbing material for attenuation of background regions surrounding the mask openings. In U.S. Pat. No. 5,288,569 issued Feb. 22, 1994 to Lin entitled FEATURE BIASSING AND ABSORPTIVE PHASE-SHIFTING TECHNIQUES TO IMPROVE OPTICAL PROJECTION IMAGING discloses a photolithography system wherein making the phase shifters absorptive facilitates a phase shifting mask system for arbitrary layouts. Combining phase shifters of different levels of absorption further enhance the improvements.
This patent also discusses another design where there is no absorber employed, only the phase shifter on the substrate carry the burden of patterning. The large phase shifter areas are printed everywhere inside and outside the features except at the edge, where due to the large phase transition, large dark line images are produced. In the small areas the edges are sufficiently close to each other so that a completely dark feature is created. Large dark images can be produced by grouping many subresolution phase shifter features closely together. Here, because the phase shifters are completely transparent as opposed to the attenuated phase shift mask to be described in the invention this particular phase shift mask system is called unattenuated (Utt) phase shift mask.
Other references related to the present technology are as follows:
U.S. Pat. No. 5,045,417 issued Sep. 3, 1991 to Okamoto entitled MASK FOR MANUFACTURING SEMICONDUCTOR DEVICE AND METHOD OF MANUFACTURE THEREOF relates to a microminiaturization technique to achieve the miniaturization and higher integration of IC chips and to the improvement of a mask used in its manufacturing process. In other words, the phase of lights transmitted through the mask is controlled within one mask pattern. Specifically, a transparent film is formed in such a manner that it covers a mask pattern along a pattern formed by magnifying or demagnifying the mask pattern or otherwise a groove is formed in a mask substrate. A phase difference of 180.degree. is generated between the lights transmitted through the mask substrate and the transparent film or the groove, causing interference with each light to offset each other. Therefore, the pattern transferred onto a wafer has an improved resolution, being used in the invention.
U.S. Pat. No. 4,902,899 issued Feb. 20, 1990 to Lin et al entitled LITHOGRAPHIC PROCESS HAVING IMPROVED IMAGE QUALITY describes a lithographic process having improved image quality by employing a mask that includes a plurality of opaque elements or transparent elements that are smaller than the resolution of the lithography to be employed in order to control the transmittance of the actinic light exposure area.
U.S. Pat. No. 4,890,309 issued Dec. 26, 1989 to Smith et al entitled LITHOGRAPHY MASK WITH A .pi.-PHASE SHIFTING ATTENUATOR discloses a lithographic system wherein the mask includes an attenuator which passes a fraction of the incident electromagnetic radiation and phase shifts the radiation relative to the radiation passing through open features of the mask by approximately an odd multiple of .pi. radians. This phase shifting of light passing through the attenuator by .pi. radians reduces the edge blurring that results from diffraction effects. The invention steepens the slope of the intensity profile at the edges of features in x-ray lithographic replication relative to the slope obtained with a conventional x-ray mask. The steeper slope is a highly significant advantage because it permits improved linewidth control.
U.S. Pat. No. 4,885,231, issued Dec. 5, 1989 to Chan entitled PHASE SHIFTED GRATING BY SELECTIVE IMAGE REVERSAL OF PHOTORESIST describes a system wherein image reversal is controlled to occur in lithographically defined regions of a positive photoresist. In that way, selective reversal of a simple holographic grating is achieved to obtain 180-degree phase shifts within lithographically defined regions of the grating. Such a phase-shifted grating is useful, for example, to provide distributed feedback in a semiconductor laser designed for single-longitudinal-mode operation.
U.S. Pat. No. 4,806,442 issued Feb. 21, 1984 to Shirasake et al entitled SPATIAL PHASE MODULATING MASKS AND PRODUCTION PROCESSES THEREOF, AND PROCESSES FOR THE FORMATION OF PHASE-SHIFTED DIFFRACTION GRATINGS relates to spatial phase modulating transparent masks comprising two or more portions having two different optical paths and their production processes are disclosed. The transparent masks are particularly useful as an exposure mask in the production of phase-shifted, distributed feedback (DFB) semiconductor lasers for a single-mode operation. A process for the formation of phase-shifted diffraction grating or corrugations which comprises exposing a substrate, through the above transparent mask, to exposure radiation is also disclosed. According to the present invention, the phase-shifted diffraction grating can be easily and directly produced with a high accuracy and reliability.